Add haml template for environment block (block_environment.html.haml in haml/html5)

Add this block to accommodate blocks like

```
[env.theorem]
--
There are infinitely many primes.
--
```

This is to facilitate the cooperation of the html and latex backends, so that both can properly render the same source text.  See the pull request for the `EnvironmentBlock` for more details.
Please note that the element following `env.` is entirely arbitary.  It can be `corollary`, `definition`, 
`joke`, whatever.